Further evolution of cellular communication systems, such as what is sometimes referred to as 5th generation (5G) cellular communication systems, will typically require bitrate performance in the order of Gb/s and signal frequency bandwidths in the order of 100 MHz in the downlink. For comparison, the maximum signal bandwidth (for a single component carrier) in a current 3GPP (3rd Generation Partnership Program) LTE (Long Term Evolution) cellular communication system is 20 MHz, i.e. a factor 5 lower. In order to find such free bandwidths, the carrier frequency may need to increase a factor 10-20 above the current (radio frequency, RF) carrier frequencies used in present 2nd, 3rd, and 4th generation (2G, 3G or 4G) cellular communications systems, which are normally in the range 1-3 GHz.
Normally, low cost and low power consumption is desirable for cellular communication devices. At the same time, there is also a desire for cellular communication devices to be capable of operating in multiple radio access technologies (RATs). A device having such multi-RAT functionality is in the following referred to as a multi-RAT device. For example, a 4G device is nominally also support operation in 2G and 3G communications systems. A reason for this is the gradual deployment of new RATs, whereby the use of a single new RAT is limiting from an end user perspective. Therefore, it is likely that new devices in the near future, supporting a 5G cellular system, also need to support legacy systems, such as one or more of 2G, 3G, and 4G systems.
A reference clock signal to a radio transceiver circuit of a cellular communication device can be provided by a crystal oscillator. The crystal oscillator can for example be designed to operate at 26 MHz, and be driven by a low-cost 32 kHz reference clock-signal generator. In order to meet constraints of low cost and low power, a certain degree of inaccuracy of the crystal oscillator must normally be accepted. The open loop uncertainty (maximum deviation from a nominal value) of the crystal oscillator frequency may be in the order of 10-15 ppm. Hence, once a cellular communication device is powered on, there is an uncertainty with respect to the reference frequency in the device, which needs to be handled by the device during an initial cell search process when the device searches for a cell to synchronize with.
In a 2G system, such as a GSM (Global System for Mobile communications) system, for which the carrier frequency is slightly below 1 GHz, the frequency uncertainty at power up of the cellular communication device can be in the order of 10-15 kHz. The FCCH (Frequency Correction CHannel) burst in GSM, which is a 67.7 kHz signal, is typically tolerant to frequency errors in that order, and typically no specific measures need to be taken during the initial cell search due to the inaccuracy of the crystal oscillator.
However, in a 3G system, such as a UMTS (Universal Mobile Telecommunications System) system, or a 4G system, such as an LTE (Long Term Evolution) system, which typically operates with carrier frequencies around 2-3 GHz, the frequency uncertainty at power up of the cellular communication device can be in the order of 20-45 kHz. At the same time, the PSCH/SSCH (Primary Synchronization CHannel/Secondary Synchronization CHannel) in a UMTS system and the PSS/SSS (Primary Synchronization Signal/Secondary Synchronization Signal) in an LTE system are typically robust for frequency errors up to 3-4 kHz. For these types of systems, so called frequency gridding can be used for the initial cell search. A frequency-gridding procedure is outlined in the following.
The actual carrier frequency of the (RF) carrier is in the following referred to as the nominal carrier frequency. With a zero frequency error in the cellular communication device, it appears to the cellular communication device that the carrier is actually located (in frequency) at this nominal carrier frequency. If, however, there is a non-zero frequency error in the cellular communication device, it appears to the cellular communication device that the carrier is located (in frequency) at some other carrier frequency. When frequency gridding is performed, the cellular communication device hypothesizes a number of such other carrier frequencies. Thereby, a set of hypothesized carrier frequencies, which may include also the nominal carrier frequency, is obtained around the nominal carrier frequency. The cellular communication device then performs a search on the hypothesized carrier frequencies until the carrier is detected. Detecting the carrier may e.g. mean detecting a synchronization channel (such as the FCCH in GSM or PSCH/SSCH in UMTS) or a synchronization signal (such as the PSS/SSS in LTE) modulated onto the carrier. Based on knowledge of the actual carrier frequency and the hypothesized carrier frequency on which the carrier was detected, the cellular communication device can then estimate the frequency error in the cellular communication device and take corrective measures in order to synchronize the reference frequency in the cellular communication device with the reference frequency of the cellular communication network.
In 3G and 4G systems, typically around 5-6 grid points are needed in order to reliably detect the PSCH/SSCH and PSS/SSS, respectively.